The invention relates to a process for recovering aluminium fluoride from fluoride containing waste materials from the production of aluminium metal (hereinafter FCWM).
Such processes are of technological importance
partly because depositing of FCWM is not acceptable for environmental reasons due to leaching out of fluorides and other ecologically harmful salts by the action of rain water PA1 partly because FCWM is an easily accessible, inexpensive starting material for the production of AlF.sub.3 which can be used in processes for the production of aluminium by electrolytic reduction of alumina. PA1 spent potlining retrieved from an aluminium electrolytic reduction cell (SPL); PA1 spent potlining ash (SPLA) produced by incinerating SPL; PA1 cold bath material, i.e. solidified electrolyte from the electrolytic reduction cells, the composition being a mixture of cryolite, chiolite, aluminium fluoride, calcium fluoride, and aluminium oxide; and PA1 other fluorine containing waste materials, such as filter dust and black sooth. PA1 burning the carbon in the potlining; PA1 raising the temperature to about 1200.degree. C.; PA1 reacting the fluorides with steam to produce hydrogen fluoride which is removed from the reaction zone as a gas. PA1 contamination of process liquors and precipitated end products due to dissolution of iron and silica and coprecipitation of iron hydroxides and silica; and PA1 environmentally unacceptable production of HCN, H.sub.2 S and phosphines in the acid leaching step. PA1 a leaching step wherein comminuted SPL is leached with an aqueous acid solution; PA1 a deironing step wherein iron is removed from the liquid phase produced in the filtration step; and PA1 a precipitation step wherein not cryolite but aluminium fluorides are precipitated from the liquid exit phase from the deironing step. PA1 the amount of fluoride extracted is increased PA1 leaching of less fine starting material is made possible and filtration problems are thereby reduced PA1 expenditure for chemicals is reduced PA1 the lay-out of the process is more simple PA1 the problems of co-precipitation of contaminating materials (such as Si, Fe and Na) are reduced PA1 there is precipitated an aluminium fluoride product, in the form of AlF.sub.2 OH hydrate, with high purity PA1 it is possible to process starting materials having extremely varying compositions PA1 it is possible to obtain a uniform end product even when the starting materials exhibit extremely varying compositions. PA1 leaching FCWM with dilute sulphuric acid in an acid leaching step to produce an acid slurry containing fluorine, sodium and aluminium dissolved in the aqueous phase; PA1 if needed, adjusting the amount of dissolved aluminium in the acid leaching step by addition of aluminium metal and/or acid soluble aluminium compounds so that the molar ratio "F"/"Al" is within the interval from about 1.8 to 2.2, where "F" designates the number of moles of leachable fluorine in the treated portion of FCWM and "Al" designates the number of moles of leachable aluminium in the treated portion of FCWM plus optionally added aluminium; PA1 adjusting the amount of sulphuric acid added in the acid leaching step so that PA1 withdrawing the acid slurry from the acid leaching step; PA1 adjusting the pH of the aqueous phase in the slurry withdrawn from the acid leaching step to a value within the range from about 3.7 to 4.1 by addition of a caustic aqueous solution, preferably of sodium hydroxide, in a silica-precipitation step, at a temperature not exceeding 60.degree. C.; and allowing silica to precipitate during a precipitation, ageing and sedimentation period of not less than 0.5-1 h; PA1 separating the reaction mixture from the silica-precipitation step into a solid phase containing the precipitated silica and the non-soluble residues of the FCWM and a purified solution containing aluminium, fluorine and sodium as main components and iron as Fe.sup.++ ions; PA1 precipitating AlF.sub.2 OH hydrate from the purified solution withdrawn from the silica precipitation step in an AlF.sub.2 OH precipitation step by increasing the temperature of the solution to and maintaining the temperature at a value within the interval from about 90.degree. C. to the boiling point of the aqueous phase, gradually increasing the pH value to about 3.7 to 4.1 by controlled continuous addition of an aqueous caustic solution, e.g. of sodium hydroxide; PA1 maintaining reducing and/or non-oxidizing operating conditions during the acid leaching step, the silica precipitation step and the AlF.sub.2 OH precipitation step in order to prevent oxidation of Fe.sup.++ to Fe.sup.+++ ; and PA1 separating and withdrawing the precipitated AlF.sub.2 OH hydrate from the AlF.sub.2 OH precipitation step. PA1 subjecting the FCWM to a preliminary leaching process in a preliminary leaching step in which FCWM is leached with water or a weakly caustic aqueous leaching solution, e.g. of sodium hydroxide, to produce a first fluorine, sodium and aluminium containing caustic solution and a solid pre-leached FCWM; PA1 transferring the pre-leached FCWM to the acid leaching step; and PA1 combining the caustic solution produced in the preliminary leaching step with the slurry from the acid leaching step in the silica-precipitation step.
Typical examples of FCWM include:
The chemical composition of FCWM may vary within wide limits, but the main components are NaF, AlF.sub.3, NaF.AlF.sub.3 double salts, including cryolite, Al metal, Al.sub.2 O.sub.3, silica and mixed silicates and sodium carbonate and hydroxide. SPL comprises further a rather large amount of carbon and refractory lining material.
Thus a typical sample of SPL may contain the following components within the indicated ranges:
______________________________________ NaF 8-12% 3NaF,AlF.sub.3 (cryolite) 12-16% Na.sub.2 CO.sub.3 3-7% NaOH 3-5% C 25-50% Al.sub.2 O.sub.3 (including Al metal) 5-25% CaF.sub.2 2-4% SiO.sub.2 1-10% ______________________________________
A typical sample of SPLA may contain the following components within the indicated ranges:
______________________________________ F (mainly as NaF, AlF.sub.3 and mixed fluorides): 20-30% Al (mainly as Al metal, Al.sub.2 O.sub.3, AlF.sub.3 and 10-25% fluorides): Na (mainly as NaF, and mixed fluorides, and 15-25% sodium oxide and carbonate): Si (mainly as silica and silicates): 5-10% C (as carbon): 2-10% Ca (mainly as oxide and fluoride): 1-2% ______________________________________
It has been suggested to recover fluorine from SPL by the so-called pyrohydrolysis processes comprising
The hydrogen fluoride might then be used for the production of AlF.sub.3.
Such processes are described in e.g U.S. Pat. No. 2,858,198.
However, the pyrohydrolysis processes have not yet been utilized on a commercial scale even though these processes have been under development for more than 10 years. The most serious problems encountered during performance of the pyrohydrolysis processes are corrosion problems caused by the presence of H.sub.2 O and HF at extreme temperatures.
It has further been suggested to recover fluorides as sodium and/or aluminium fluorides from e.g. SPL by wet processes.
For instance, it has been proposed to recover fluoride values such as cryolite from SPL material by treating finely divided SPL material with an aqueous caustic solution to effect a reaction between the fluoride values or cryolite therein with sodium hydroxide to yield water soluble sodium fluoride and water soluble sodium aluminate and then to acidify the resulting solution, e.g. by introduction of carbon dioxide, to precipitate cryolite therefrom. This method of recovering cryolite from cell lining material suffers from the disadvantage of requiring a considerable amount of excess sodium hydroxide which is costly while the time necessary to effect the reaction is somewhat prolonged with the further disadvantage that it is accompanied by much foaming due to gas formation.
Another alkaline leaching process in which fluorine is recovered as NaF is described in European patent specification 117 616.
In this process the cryolite-containing SPL is treated with an aqueous caustic soda solution in order to break down the cryolite into sodium fluoride and sodium aluminate.
According to one aspect the caustic soda solution contains from 200 to 400 g/l caustic (calculated as Na.sub.2 CO.sub.3). The liquor is separated from the solid residue after the treatment and then the sodium fluoride is extracted from the solid residue by contacting with water (including a dilute aqueous sodium fluoride solution).
According to another aspect the caustic soda solution contains from 20 to 70 g/l caustic (calculated as Na.sub.2 CO.sub.3). The caustic soda solution is separated from the undissolved residue, and concentrated by evaporation thereby causing the precipitation of solid sodium fluoride from said solution. Finally, the precipitated sodium fluoride is separated from the caustic soda solution.
However, the recovery of aluminium fluorides from FCWM is rather difficult when basic leaching methods are used because precipitation of aluminium and fluoride from an aqueous sodium containing solution will inevitably lead to cryolite as precipitated product, unless special and rather complicated measures are taken.
It has further been suggested to process SPLA and SPL by processes involving treatment with sulphuric acid.
In U.S. Pat. No. 4,900,535 there is described a process which involves treating SPLA with concentrated sulphuric acid at elevated temperatures, whereby the fluoride values are reacted to and withdrawn as HF.
A similar process for treating SPL is described in U.K. patent application 056,422 A.
These processes exhibit essentially the same inconveniences as the pyrohydrolysis processes and have not been carried out on a commercial scale.
It has also been suggested to recover Al and F from SPL by acid treatment processes operating at more moderate conditions in which fluorine is not recovered as gaseous HF.
In U.S. Pat. No. 2,186,433 there is described a process for recovery of aluminium and fluorine compounds, actually cryolite, from SPL, wherein the aluminium and fluorine compounds contained in the SPL are brought into solution by treating the SPL with a diluted aqueous leaching solution containing an acid selected from the group consisting of hydrochloric acid and sulphuric acid, and containing an aluminium salt of said acids. The aluminium and fluorine compounds contained in the extract so obtained are then precipitated as cryolite by the addition of hydrofluoric acid and an alkali salt.
However, these acid leaching processes present problems of their own which mainly consist in
Other acid leaching processes have been suggested comprising
A variant of this process is described in U.S. Pat. No. 4,889,695 which relates to a two step leaching process wherein SPL is leached with a caustic solution, preferably having a concentration of about 14 g/L NaOH, in a first leaching step and subsequently with an acid aqueous solution of aluminium sulfate and sulfuric acid. After removal of iron from the liquor from the acid leaching step in a separate deironing step the two leaching liquors are united and aluminium fluoride is precipitated under carefully controlled conditions.
According to U.S. Pat. No. 4,597,953 improved fluoride recovery from SPL should be obtained by leaching with an aqueous acid leaching solution having an aluminium sulfate/sulfuric acid ratio in the range from about 0.75 to 1.0.
The precipitation of aluminium fluoride from a solution produced in an acid leaching method is complicated by the presence of "foreign ions", i.e. ions different from Al and F in the liquor. Sodium ions constitute a special problem because sodium and aluminium double salts, e.g. Chiolite and/or Cryolite, may be precipitated instead of aluminium fluoride in the precipitation step.
According to U.S. Pat. No. 4,508,689, an improved yield of sodium-poor aluminium fluoride hydroxide should be obtained when the precipitation is carried out from an acid solution of ions of Al, F and Na which is incompletely neutralized with a basic solution of a sodium compound to increase the pH into the range of about 5.0 to 5.6. According to the specification incomplete neutralization to pH values below about 5 results in a significant loss of fluorine values.
It is the object of the present invention to provide an improved wet process for recovering aluminium fluoride from FCWM, in particular from SPL and SPLA, whereby compared with the prior art